ULTRAVIOLET RESONANCE RAMAN EXAMINATION OF THE LIGHT-INDUCED PROTEIN STRUCTURAL-CHANGES IN RHODOPSIN ACTIVATION

Ultraviolet resonance Raman (UVRR) spectra of rhodopsin and its metarh odopsin I and metarhodopsin II photointermediates have been obtained t o examine the molecular mechanism of G-protein-coupled receptor activa tion. Spectra were acquired using a single-pass capillary flow techniq ue in combination with a Littrow prism UV prefilter detection system. The UVRR difference spectra between rhodopsin and metarhodopsin I exhi bit small differences assignable to tyrosine residues and no differenc es due to tryptophan. The UVRR difference spectra between rhodopsin an d metarhodopsin II exhibit significant differences for vibrations of b oth tryptophan and tyrosine residues. Most importantly, there is an in tensity decrease of the totally symmetric tryptophan modes at 759, 100 8, and 1545 cm(-1), an intensity decrease of the tryptophan W7 band at 1357 cm(-1), and a frequency shift of the tryptophan W17 band from 88 5 to 892 cm(-1) These difference features are assigned to one or more tryptophan residues that reside in a hydrophobic, weakly hydrogen-bond ing environment in rhodopsin and that are transferred to a less hydrop hobic, non-hydrogen-bonding environment during rhodopsin activation. T he available evidence suggests that Trp(265) makes a dominant contribu tion to the tryptophan features in this difference spectrum. These res ults are interpreted with a model for rhodopsin activation in which re tinal isomerization alters the interaction of Trp(265) With the ionone ring of the retinal chromophore.